The present invention relates to a core glass for the preparation of a preform for an optical fiber, especially an optical fiber for the transmission of ultraviolet radiation, obtained by flame hydrolysis of a silicon compound, deposition of finely granular SiO2 on a substrate with direct vitrification and formation of a synthetic quartz glass.
The invention furthermore relates to a preform for an optical fiber, especially for an optical fiber for the transmission of UV radiation, with a core glass of synthetic quartz glass, obtained by flame hydrolysis of a silicon compound, deposition of finely granular SiO2 on a substrate with direct vitrification and formation of a synthetic quartz glass which is enveloped in a jacket glass.
The invention furthermore relates to a method for the production of a core glass for a preform for an optical fiber, especially for an optical fiber for the transmission of UV radiation, comprising the production of synthetic quartz glass by flame hydrolysis of a silicon compound, deposition of finely granular SiO2 on a substrate with direct vitrification and formation of the synthetic quartz glass.
The invention furthermore relates to a method for the manufacture of an optical fiber, especially an optical fiber for the transmission of UV radiation, by drawing from a preform which comprises a core glass of synthetic quartz glass which is formed by flame hydrolysis of a silicon compound, deposition of finely granular SiO2 on a substrate with direct vitrification and formation of the synthetic quartz glass.
Preforms for optical fibers generally have a core which is enveloped by a jacket of a material with a lower index of refraction. Preforms are also known which consist only of a so-called core rod of core material, the jacket or a portion of the jacket being applied to the core material when the fiber is drawn from the preform. For the manufacture of preforms for optical fibers from synthetic quartz glass, essentially three methods have become established which are referred to in the technical literature as VAD processes (vapor phase axial deposition), OVD processes (outside vapor-phase deposition), and MCVD processes (modified chemical vapor deposition). Furthermore, the production of preforms by the so-called rod-in-tube technique is known. In all these methods a core glass of synthetic quartz glass is generally made by the flame hydrolysis of a silica containing compound by producing SiO2 particles and depositing them and vitrifying them on a substrate. The substrate can be, for example, a quartz glass tube which consists of a jacket glass. The vitrification of the SiO2 particles can be performed directly during its deposition on the substrate (referred to hereinafter as direct vitrification), or in a separate sintering process as in the case of the so-called xe2x80x9csoot process.xe2x80x9d Both variant vitrifications result in a dense, transparent, high-purity quartz glass.
On account of their porosity, so-called soot bodies are simply cleaned, doped or otherwise treated prior to vitrification. On the other hand, directly vitrified synthetic quartz glass has advantages for some applications in connection with the transmission of short-wave UV radiation. Due to the presence of hydrogen and oxygen during the flame hydrolysis, quartz glass made by direct vitrification generally contains a relatively high OH content and a certain concentration of hydrogen.
Depending on the method, the jacket glass is produced in a separate process (OVD, MCVD, plasma process, rod-in-tube technique), or the jacket glass and the core glass are produced simultaneously as in the so-called VAD process. To change the index of refraction of the quartz glass, a dopant is usually added, such as germanium, for example, to increase the index, or fluorine and boron to lower the index of refraction.
The present invention sets out from these long-known processes for the production of a preform, the core glass of the preform being made by direct vitrification. By the heating and drawing of the preform thus prepared optical fibers are obtained from it.
Such optical fibers are used not only for the transmission of information in the form of optical signals in communication technology, but also increasingly for the transmission of high-energy UV radiation, as for example in medical technology, in material machining, in ultraviolet spectroscopy or in microlithography apparatus for the production of highly integrated circuits in semiconductor chips. The illumination systems of modern microlithography apparatus are equipped with excimer lasers which emit high-energy, pulsed UV radiation of a wavelength of 248 nm (KrF lasers) or 193 nm (ArF lasers). It is known that such short-wave UV radiation can produce structural defects in the quartz glass of the optical fibers and resultant absorptions. For example, an excess oxygen defect, in which a non-bridge building oxygen atom is present (a so-called NBOH center) results in a relatively broad absorption band at a wavelength of about 265 nm. A defect in which only three oxygen atoms (instead of four) are bound to a silicon atom, and which is called an Exe2x80x2 center, produces an absorption band around 215 nm. A review of structural defects in quartz glass is given by David L. Griscom in xe2x80x9cDefect Structure of Glassesxe2x80x9d, J. Non-Cryst. Solids, 73 (1985), pages 55-77.
The influence of the chemical composition of quartz glass on the damage that can be done by irradiation with high-energy UV light is described, for example, in European Patent Application A1 401 845. Accordingly, a high stability in radiation was found in high-purity quartz glass containing a relatively high OH content of 100 wt.-ppm to about 1000 wt.-ppm and at the same time a relatively high hydrogen concentration of at least 5xc3x971016 molecules per cm3 (with respect to the volume of the quartz glass). The good influence of hydrogen on stability in radiation can be explained by the fact that it can contribute to the healing of defects and thus to a slower increase of the adsorption caused by radiation. On account of this action of hydrogen it is recommended in European Patent Application A1 401 845 that optical components which must satisfy stringent requirements in regard to stability under radiation be charged with hydrogen.
In European Patent Application EPA1 590 199 a description is given of core glass of the kind for the production of an optical fiber for the transmission of high-energy UV light a preform made with the use of the core glass, a method for the production of the core glass, and a generic process for the production of such an optical fiber. The prior-art core glass is a pure synthetic quartz glass which is made by the flame hydrolysis of methyl trimethoxysilane. The core glass is substantially free of chlorine, its hydroxyl group content (OH content) is between 10 and 1000 ppm, and it contains fluorine in a concentration ranging between 50 and 5000 ppm. To prepare a preform for a so-called graded index fiber a core glass rod is provided with a tube of core glass by the xe2x80x9crod-in-tubexe2x80x9d technique wherein the jacket glass consists of fluorine-doped or boron-doped quartz glass. An optical fiber is drawn from the preform by heating the preform to about 2000xc2x0 C.; it softens beginning at one end, and the fiber is drawn from the softened part. The known optical fiber shows, in comparison to other fibers, good stability under high-energy UV radiation. For applications in which an especially slow increase of the radiation-induced absorption, a low transmission loss, and a good long-term stability are important, the known optical fiber, however, is inadequate.
The invention is therefore addressed to the problem of optimizing the stability of a core glass for the production of a preform, obtained by direct vitrification, under ultraviolet radiation, especially under high-energy ultraviolet radiation of a wavelength of 250 nm and shorter, with a view to lower absorption, slower increase of absorption and greater long-term stability.
The invention is furthermore addressed to the problem of making available a preform made by the use of the core glass, from which an optical fiber can be drawn which has a higher stability than known fibers have against ultraviolet radiation of a wavelength of 250 nm and shorter.
Further purposes of the invention are to devise a simple method for the production of such a core glass, plus an optical fiber with optimized stability against ultraviolet radiation.
With regard to the core glass, this problem is solved according to the invention, setting out from the core glass described in the beginning, in that the quartz glass has a hydrogen content of less than 1xc3x971018 molecules/cm3.
The core glass is used to prepare a preform from which optical fibers are drawn. To draw fibers the preform is heated and deformed plastically. It has been found that in this hot deformation of the preform, so-called precursor defects are produced, from which other structural defects can develop by radiation with short-wave UV light. One precursor defect is, for example, a bond between silicon and hydrogen (hereinafter called a Sixe2x80x94H bond). Many of these Sixe2x80x94H bonds occur in the fiber when the core glass of the preform consists of quartz glass with a hydrogen content of more than 1xc3x971018 molecules/cm3. This can be explained by the fact that the high temperatures occurring when the fibers are drawn and the severe plastic deformation of the quartz glass results in structural defects, such as free bonds of silicon, are formed, which due to the presence of hydrogen become saturated, with the formation of Sixe2x80x94H bonds. The mechanism can be represented schematically by the following formula: 
The saturation of free bonds forming when fibers are drawn, due to the hydrogen present, leads to a high precursor concentration (Sixe2x80x94H bonds) in the fiber core. In the publication of J. E. Shelby, xe2x80x9cMolecular diffusion and solubility of hydrogen isotopes in quartz glassxe2x80x9d, J. Appl. Physics, Vol. 48 (1977), No. 8, pages 3387 ff, it is described how Sixe2x80x94H bonds can form at high temperatures in quartz glass containing hydrogen. It is true that Sixe2x80x94H bonds do not themselves absorb in the relevant UV wavelength range; the bonds, however, are relatively weak and, upon subsequent irradiation with short-wave ultraviolet light, they are easily broken (in a so-called xe2x80x9csingle photon processxe2x80x9d) with the formation of absorbing Exe2x80x2 centers. This process is described in detail in xe2x80x9cComparison of the influence of the fictive and the annealing temperature on the UV transmission properties of synthetic fused silicaxe2x80x9d by V. Uhl et al. in Appl. Phys. A, 65 (1997), pages 457-462. Since Exe2x80x2 centers absorb in the ultraviolet wavelength range, this conversion mechanism, which is represented schematically by the following formula, has a negative effect on the radiation stability of the fibers.
xe2x89xa1Sixe2x80x94H+hxcexdxe2x86x92xe2x89xa1Si.+H (Exe2x80x2 center) 
This disadvantage is not shown, however, by an optical fiber which has been drawn from a preform containing a core glass of the invention. For this purpose the hydrogen content of the core glass prior to hot deformation in the drawing of the fiber is adjusted to a value below 1xc3x971018 molecules/cm3. It has been found that in this case the precursor concentration in the fiber core is decidedly lower after the fiber is drawn. Evidently the formation of Sixe2x80x94H bonds in the drawing of the fiber is suppressed due to the low hydrogen content. The behavior of such an optical fiber in connection with high-energy UV light is characterized by a definitely slower initial rise of the induced absorption. This can be attributed to the low precursor concentration in the fiber core.
The core glass of the invention is made by xe2x80x9cdirect vitrification.xe2x80x9d Fibers from preforms whose core glasses were produced by the xe2x80x9csoot methodxe2x80x9d or by the xe2x80x9cplasma methodxe2x80x9d showed a so-called xe2x80x9cdrawing bandxe2x80x9d in the UV wavelength range around 265 nm, this occurred whenever the core glass otherwise was the same as the core glass of the invention. This might be connected with the specific conditions of manufacture.
The hydrogen content is determined by a Raman measurement which has been described by Khotimchenko et al., xe2x80x9cDetermining the Content of Hydrogen Dissolved in Quartz Glass Using the Methods of Raman Scattering and Mass Spectrometryxe2x80x9d in xe2x80x9cZhurnal Prikladnoi Spektroskopiixe2x80x9d Vol. 46, No. 6 (1987), pages 987 to 991. The unit of measurement, xe2x80x9cmolecules/cm3xe2x80x9d relates to the number of hydrogen molecules per cm3 of quartz glass. It would be optimum if the hydrogen content over the entire radial cross section of a cylindrical core glass would be below the stated concentration. Often the hydrogen content over the radial cross section of the core glass, however, is not uniformly distributed. Depending on the conditions of production, maxima of the hydrogen concentration can be in the marginal area or in the central area of the core glass. Under certain circumstances, such maxima, especially in the marginal areas, can be above 1818 molecules/cm3 and yet be harmless. It is assumed that, for a core glass to be suitable in the meaning of this invention, the hydrogen concentration in its central area is essential. Therefore, in the meaning of the present invention, the hydrogen content is understood to be the hydrogen and/or deuterium concentration determined by a Raman measurement in the center of the radial core glass cross section, the diameter of the measuring beam being less than 1 mm. The detection limit for the hydrogen content in this method of measurement is at this time approximately 2xc3x971016 molecules/cm3 by the conventional measurement times. In the case of very long integration periods (for example 24 hours or longer) the accuracy of measurement can be increased to about 2xc3x971015 molecules/cm3.
An optical fiber that is drawn from a preform in which the core glass consists of quartz glass and has a hydrogen content of no more than 1xc3x971017 molecules/cm3 proves to be desirable with regard to UV damage.
It has proven to be advantageous if the quartz glass has a hydroxyl group content of at least 100 wt.-ppm. Hydroxyl groups improve the resistance of the core glass to radiation. The hydroxyl group content in the meaning of this invention is measured, like the hydrogen content, in the central part of the core glass, although the OH content is determined by spectroscopy.
A preferred core glass consists of quartz glass having a hydrogen content of at least 2xc3x971015 molecules/cm3. From a preform with a core glass in which the hydrogen content is set below this amount an optical fiber is obtained which at approximately 265 nm has an absorption band which gets more pronounced under UV radiation. This absorption band indicates NBOH centers. From this it is to be concluded that the hydrogen content of the quartz glass has, with regard to damage by UV radiation, an optimum that is between 1xc3x971018 molecules/cm3 and 2xc3x971015 molecules/cm3, preferably between 1xc3x971017 molecules/cm3 and 2xc3x971015 molecules/cm3. One possible explanation of this is that it takes a certain hydrogen content in the core glass of the preform to prevent the formation of NBOH centers and/or defects that are precursors of NBOH centers. The damage to the fibers that has been described becomes undesirably observable only in the transmission of UV radiation of a wavelength within the range of the absorption bands (e.g., 248 nm, but not at 193 nm). Anyway, this disadvantageous effect of a low hydrogen content can be compensated by an additional measure as it will be explained further on in connection with a method for the production of an optical fiber.
With regard to the preform for an optical fiber, the problem referred to above is solved according to the invention, setting out from the preform described in the beginning, by the fact that the core glass has a hydrogen content of less than 1xc3x971018 molecules/cm3.
An optical fiber is drawn from the preform. To draw the fiber, the preform is heated and thus plastically deformed. It has been found that, by this hot shaping of the preform, so-called precursor defects are produced in the quartz glass, from which other structural defects can develop upon irradiation with short-wave UV radiation. A precursor defect, for example, is a bond between silicon and hydrogen (hereinafter called an Sixe2x80x94H bond). Many of these Sixe2x80x94H bonds occur in the fiber if the core glass of the preform consists of quartz glass with a hydrogen content of more than 1xc3x971018 molecules/cm3. This can be explained by assuming that the high temperatures involved in fiber drawing and the severe plastic deformation of the quartz glass results in structural defects, as for example free bonds of silicon which become saturated by the hydrogen present, with the formation of Sixe2x80x94H bonds. The mechanism can be represented schematically by the following formula: 
The saturation of free bonds that develop in fiber drawing due to the presence of hydrogen leads to a high precursor (Sixe2x80x94H bonds) concentration in the fiber core. In the publication of J. E. Shelby, xe2x80x9cMolecular diffusion and solubility of hydrogen isotopes in vitreous silicaxe2x80x9d, J. Appl. Physics Vol. 48 (1977) No. 8, pages 3387 ff, it is described that, in quartz glass containing hydrogen, Sixe2x80x94H bonds can form at high temperatures. Sixe2x80x94H bonds themselves do not absorb in the relevant UV wavelength range; the bonds, however, are relatively weak and, in subsequent irradiation with short-wave UV light, they can easily (in a so-called xe2x80x9csingle-photon processxe2x80x9d) break up with the formation of absorbing Exe2x80x2 centers. This process is further described in xe2x80x9cComparison of the influence of the fictive and the annealing temperature on the UV transmission properties of synthetic fused silicaxe2x80x9d by V. Uhl et al., in Appl. Phys. A, 65 (1997), pages 457-462. Since Exe2x80x2 centers absorb in the UV wavelength range, this transformation mechanism has a negative effect on the resistance of optical fibers to radiation.
This disadvantage is not, however, shown by an optical fiber which has been drawn from a preform of the invention. The hydrogen content of the core glass of the preform is adjusted to a level below 1xc3x971018 molecules/cm3 before the hot forming operation. It has been found that, in this case, the precursor concentration in the fiber care is definitely lower after the fiber is drawn. Apparently, the formation of Sixe2x80x94H bonds when fiber is drawn is suppressed on account of the low hydrogen content. The damage resistance of such an optical fiber to high-energy UV radiation is characterized by a definitely slower initial increase of the induced absorption. This can be attributed to the low precursor concentration in the fiber core.
The core glass of the preform of the invention is produced by xe2x80x9cdirect vitrification.xe2x80x9d Fibers from preforms whose core glasses were made by the xe2x80x9csoot processxe2x80x9d or by xe2x80x9cplasma processesxe2x80x9d showed in the UV wavelength range around 265 nm a so-called xe2x80x9cdrawing barrierxe2x80x9d which occurred even when the core glass otherwise corresponded to the above-described core glass according to the invention. This might be connected with the specific conditions of production.
The determination of the hydrogen content is performed similar to the determination described above which is performed on the core glass of the invention.
Especially good damage resistance is shown by an optical fiber which is drawn from a preform in which the core glass consists of quartz glass having a hydrogen content of no more than 1xc3x971017 molecules/cm3.
It has proven advantageous that the quartz glass has a hydroxyl group content of at least 100 wt.-ppm. The hydroxyl group content improves the radiation resistance of the preform and of a fiber drawn from it. The determination of the hydroxyl group content of the preform is performed similarly to the above-explained determination of the hydroxyl group content in the core glass.
In an especially preferred embodiment, the quartz glass of the core glass has a hydrogen content of at least 2xc3x971015 molecules/cm3. From a preform with a core glass in which the hydrogen content is set below this level, an optical fiber is obtained which at about 265 nm has an absorption band which gets more pronounced under UV radiation. This absorption band indicates NBOH centers. From this it can be concluded that the hydrogen content of the quartz glass has an optimum resistance to damage by ultraviolet radiation, which is situated between 1xc3x971018 molecules/cm3 and 2xc3x97105 molecules/cm3, preferably between 1xc3x971017 molecules/cm3 and 2xc3x971015 molecules/cm3. One possible explanation for this is that a certain hydrogen content in the core glass of the preform is necessary in order to prevent the formation of NBOH centers and/or precursor defects for NBOH centers when fibers are drawn. The damage to the fibers to damage as described becomes undesirably apparent only in the transmission of UV radiation of a wavelength situated in the range of the absorption band (e.g., 248 nm, but not at 193 nm). In any case, this disadvantageous effect of a low hydrogen content can be compensated by an additional measure, as it will be explained further below in connection with a method for the production of an optical fiber.
In a case in which the preform has a jacket of synthetic quartz glass, the latter will also advantageously have a hydrogen content of less than 1xc3x971018 molecules/cm3, preferably of less than 1xc3x971017 molecules/cm3. This will reduce the danger of hydrogen diffusing from the jacket into the core glass as the fiber is drawn.
As regards the process of producing a core glass of a preform for an optical fiber, the problem stated above is solved, setting out from the method described above, by adjusting the hydrogen content of the quartz glass to a level below 1xc3x971018 molecules/cm3.
Due to the presence of hydrogen during the production of the core glass, the latter will contain hydrogen after vitrification. In a case in which the hydrogen concentration is above the above-given maximum limit of 1xc3x971018 molecules/cm3, it is necessary to drive the hydrogen out of the core glass.
In the method of the invention, the hydrogen content of the core glass is adjusted to a level below 1xc3x971018 molecules/cm3 before a fiber is drawn from the preform. To adjust the hydrogen content, the core glass can be subjected to a separate treatment; the adjustment, however, can also be made in connection with a process step in the course of the production of the preform (for example in the production of the jacket glass by the OVD method), wherein the hydrogen content of the entire preform (core glass plus jacket glass) is usually influenced.
With regard to the effects of the targeted hydrogen content on the resistance to damage in an optical fiber that is drawn from a preform using the core glass, see the explanations above concerning the core glass of the invention. An important idea of the invention is to keep the concentration of Sixe2x80x94H bonds in the fiber core as low as possible, since they can act as precursor defects and impair the resistance of the fiber to damage by UV radiation.
A method has proven to be especially advantageous in which the hydrogen content of the quartz glass is adjusted to a value below 1xc3x971017 molecules/cm3. An optical fiber which is obtained from a preform thus prepared is characterized by an especially good long-term stability under high-energy UV radiation, and an especially low level of absorption induced by UV radiation.
In this connection a method has also proven practical in which the hydroxyl group content of the quartz glass is adjusted to at least 100 wt.-ppm. Hydroxyl groups improve the stability of the core glass under radiation. For the adjustment of the hydroxyl group content a separate treatment is usually unnecessary, since usually it is above the stated lower limit in the course of the manufacture of the core glass. The hydroxyl group content is measured, like the hydrogen content, in the central area of the core glass, in which case, however, the hydroxyl group content is determined spectroscopically.
In a preferred process variant, the hydrogen content of the quartz glass is adjusted to a level of at least 2xc3x971015 molecules/cm3. From a preform with a core glass in which the hydrogen content is adjusted below this level an optical fiber is obtained which may have undesirable resistance to UV damage. See the above explanations given in connection with the core glass of the invention.
For the sake of resistance to damage by UV radiation, the hydrogen content of the quartz glass has an optimum which lies between 1xc3x971018 molecules/cm3 and 2xc3x971015 molecules/cm3, preferably between 1xc3x971017 molecules/cm3 and 2xc3x971015 molecules/cm3. In a core glass with a hydrogen content below the stated bottom limit, therefore, it is advantageous to enrich it with hydrogen. Enriching the core glass with hydrogen can be performed in a separate treatment step, or on the completed preform, or in connection with a process step.
A possible explanation of the observed effect is that it requires a certain content of hydrogen in the core glass of the preform in order to avoid the formation of NBOH centers and/or precursor defects for NBOH centers when the fiber is drawn. NBOH centers produce a broad absorption band at a wavelength of about 265 nm. They therefore make themselves undesirably manifest, especially in the case of the transmission of UV radiation of a wavelength within this absorption band, yet not as manifest in the transmission of UV radiation of a wavelength below 200 nm. Anyway, this disadvantageous effect produced by too low a hydrogen content in the core glass can be compensated by an additional measure, as will be explained further below in connection with a method for the manufacture of an optical fiber.
In a preferred procedure, a quartz glass containing hydrogen is produced, in which the adjustment of the hydrogen content entails a hydrogen reduction treatment to which the quartz glass containing hydrogen is subjected, resulting in the formation of the synthetic quartz glass. Owing to the presence of hydrogen in the case of manufacture by flame hydrolysis, the quartz glass after vitrification contains hydrogen. It is therefore generally necessary to drive the hydrogen out of the quartz glass. By the hydrogen reduction treatment the hydrogen content can be repeatably adjusted to a second concentration after setting out from an initial high concentration. The preforms thus made therefore yield optical fibers with a definite resistance to UV damage.
The quartz glass is subjected to the hydrogen reduction treatment usually in a separate process step. The hydrogen reduction treatment can also, however, be performed on the complete preform (core glass+jacket glass), or can take place in connection with a process step for the preparation of the preform, for example during the deposition of jacket glass onto the core glass.
The adjustment of the hydrogen content advantageously comprises a thermal treatment of the quartz glass, a vacuum treatment, and/or treatment in a chemically reactive atmosphere. The said variant treatments can be used alternatively or cumulatively. But a thermal treatment (referred to hereinafter as xe2x80x9cannealingxe2x80x9d) has proven to be especially effective. The annealing is performed in a hydrogen-free atmosphere, for example under inert gas or in a vacuum, in order to reduce the hydrogen content of the quartz glass, or in an atmosphere containing hydrogen under standard pressure or excess pressure, in order to increase the hydrogen content of the quartz glass. Hydrogen is diffused in quartz glass more rapidly by the temperature increase in annealing. As hydrogen is driven out, first the regions near the surface are deprived, and later the central regions of the core glass. It is therefore necessary to make sure that the hydrogen content is sufficiently removed, especially in that region which is most severely stressed optically when the fibers made from the core glass are used as intended; generally that region is precisely the central regions of the fiber. On the other hand, by annealing the core glass the hydrogen content can be reduced below the above-mentioned minimum of 2xc3x971015 molecules/cm3, which can prove to be disadvantageous.
The annealing of quartz glass for optical components is a frequently used procedure which usually serves to relieve mechanical stress which impair the optical properties of the glass. The annealing of the core glass here proposed for adjusting the hydrogen content differs from the known annealing processes in its purpose, performance and outcome. Thus the core glass is annealed preferably at a temperature of at least 600xc2x0 C., the heating time being determined by the thickness of the core glass and the hydrogen content to be established. The latter amounts, according to the invention, to less than 1xc3x971018 molecules/cm3, preferably no more than 1xc3x971017 molecules/cm3 and at least 2xc3x971015 molecules/cm3.
With regard to the process for making an optical fiber, the problem stated above is solved by the invention, setting out from the drawing process mentioned above, in that the core glass is adjusted before drawing to a hydrogen content of less than 1xc3x971018 molecules/cm3.
On account of the presence of hydrogen during the process of making the core glass, the latter can contain hydrogen after vitrification. If the hydrogen concentration is above the above-named maximum limit of 1xc3x971018 molecules/cm3, it is necessary to drive the hydogen out of the core glass. In the method of the invention, the hydrogen content of the core glass is adjusted to a level below 1xc3x971018 molecules/cm3 before a fiber is drawn from the preform. To adjust the hydrogen content the core glass can be subjected to a separate treatment; the adjustment, however, can also be made in connection with a process step during the production of the preform, wherein the hydrogen content of the entire preform (core glass plus jacket glass) is usually affected.
Regarding the effects of the hydrogen content on UV damage of an optical fiber that is drawn from a preform made from the core glass, see the explanations given above concerning the core glass of the invention.
A process has proven especially advantageous in which the hydrogen content of the core glass is adjusted, prior to the drawing of the optical fiber, to a level below 1xc3x971017 molecules/cm3. An optical fiber thus made is characterized by an especially good long-term stability against high-energy UV radiation and an especially low level of absorption induced by UV radiation.
In this respect, a process also has proven practical in which the hydroxyl group content of the core glass before drawing is adjusted to at least 100 wt.-ppm. Hydroxyl groups improve the radiation resistance of the core glass. Owing to the method of manufacture no separate treatment is necessary for the adjustment of the hydroxyl group content, since it usually proves to be above the stated bottom limit during the production of the core glass. The hydroxyl group content is, like the hydrogen content, measured in the central region of the core glass, although the hydroxy group content is determined spectroscopically.
In a preferred process variant, the hydrogen content of the core glass is adjusted to a level of at least 2xc3x971015 molecules/cm3 before drawing the fiber. From a preform with a core glass in which the hydrogen content is adjusted below this level, an optical fiber is obtained which can exhibit poor resistance to UV damage. See the above explanations in connection with the core glass of the invention.
For resistance to damage from UV radiation, the hydrogen content of the core glass is adjusted before drawing to an optimum that is between 1xc3x971018 molecules/cm3 and 2xc3x971015 molecules/cm3, preferably between 1xc3x971017 and 2xc3x971015 molecules/cm3. In a core glass with a hydrogen content below the stated bottom limit it is therefore advantageous to enrich it with hydrogen before the fiber drawing process. The enrichment of the core glass with hydrogen can be performed in a separate treatment, or on the complete preform, or in connection with a step in the process of making the preform.
It is found that an excessively low hydrogen content in the core glass before the fiber is drawn can impair the UV damage resistance of the fiber. A possible explanation of this effect is that it requires a certain hydrogen content in the core glass of the preform if the formation of NBOH centers and/or precursor defects for NBOH centers during the drawing are to be avoided. NBOH centers produce a broad absorption band at a wavelength of approximately 265 nm. Therefore they make themselves undesirably apparent especially in the transmission of UV radiation of a wavelength within this absorption band, but less in the case of UV radiation of a wavelength below 200 nm. This disadvantageous effect can be compensated by an additional measure which will be further explained below.
A process shows itself to be especially effective, in which a quartz glass containing hydrogen is produced, in which the adjustment of the hydrogen content comprises a hydrogen reduction treatment to which the hydrogen-containing quartz glass is subjected during the formation of the core glass. Due to the presence of hydrogen in the formation process, the core glass contains hydrogen after vitrification. It can therefore be necessary to drive the hydrogen out of the core glass. By the hydrogen reduction treatment, the hydrogen content can be adjusted repeatably prior to the drawing of the fiber from an initial high concentration to a second, lower concentration. The preforms thus produced therefore yield optical fibers having a defined resistance to UV damage.
The core glass is subjected to the hydrogen reduction treatment usually in a separate process step. The hydrogen reduction treatment can also, however, be performed on the complete preform (core glass plus jacket glass), or in connection with a process step for the preparation of the preform. For example, hydrogen is driven simultaneously from the core glass and from the jacket glass, and thus the harmful effect of this hydrogen in the following fiber drawing is reduced.
The adjustment of the hydrogen content advantageously comprises a heat treatment of the core glass, a treatment under vacuum, and/or a treatment in a chemically reactive atmosphere. The variant treatments stated can be applied alternatively or cumulatively. However, a heat treatment (called xe2x80x9cannealingxe2x80x9d hereinafter) has proven especially effective. The annealing is performed in a hydrogen-free atmosphere, e.g., under inert gas or in a vacuum, in order to reduce the hydrogen content of the core glass, and in a hydrogen-containing atmosphere under standard pressure or excess pressure in order to increase the hydrogen content of the core glass. Due to the temperature elevation during annealing the hydrogen diffuses especially rapidly in quartz glass. As hydrogen is driven out, first the regions near the surface are depleted and later the center regions are depleted of hydrogen.
It is therefore necessary to see that the hydrogen content is removed sufficiently especially in the region which is most greatly stressed optically by the intended use of the fibers made from the core glass; that is generally precisely the central regions of the fiber. On the other hand care must be taken to see that the hydrogen content is not set below the above-stated bottom limit of 2xc3x971015 molecules/cm3, since this too can prove disadvantageous.
It has proven especially advantageous to charge the fibers with hydrogen after drawing them from the preform. The hydrogen charging has a positive effect on the UV stability of the optical fiber, regardless of whether the fiber is or is not drawn from a preform according to the invention. In an optical fiber prepared according to the invention, however, hydrogen charging leads to an especially evident improvement of resistance to damage by UV radiation. On account of the high concentration of precursor centers in the hydrogen-charged fibers according to the state of the art, they are quickly transformed at the beginning of UV radiation to absorbing defect centers (Exe2x80x2 and NBOH centers), which leads to rapid initial damage. In hydrogen charged fibers according to the invention, however, only a few UV absorbing defect centers are formed on account of the low precursor concentration at the beginning of UV irradiation, and they can effectively be desaturated by the molecular hydrogen that is present. This desaturation can be represented as follows:
xe2x89xa1Si.+xc2xdH2xe2x86x92xe2x89xa1Sixe2x80x94H 
xe2x89xa1SiO.+xc2xdH2xe2x86x92xe2x89xa1Sixe2x80x94OH 
This applies especially to fibers which have been drawn from a preform with a core glass which prior to drawing had a hydrogen content that is below a hydrogen content that is optimum as regards UV damage. As it has already been mentioned above, the optimum hydrogen content of the core glass is to be assumed to be between 1xc3x971018 molecules/cm3 and 2xc3x971015 molecules/cm3, preferably between 1xc3x971017 molecules/cm3 and 2xc3x971015 molecules/cm3. In the case of a hydrogen content below this optimum, NBOH centers evidently form increasingly when the fiber is drawn and produce a broad absorption band at a wavelength of about 265 nm which is further intensified by UV radiation. Surprisingly, this disadvantageous effect can be eliminated completely or nearly completely by charging the fiber with hydrogen after its production. This can be explained by the fact that the range around the optimum hydrogen content of the core glass, as it is explained above, is expanded by the later hydrogen charging of the fiber towards lower hydrogen contents.
Preferably, therefore, the fiber is formed with a core, and at least in the fiber core the hydrogen charging establishes a hydrogen content of at least 1xc3x971018 molecules/cm3.